Getting access to clean drinking water is an ongoing problem for people in developing countries. And even cities that have good water-treatment systems are looking for better ways to deliver safer, cleaner water. Now an international research team has developed a photocatalyst that promises quick, effective water disinfection using sunlight or artificial light. What's more, the photocatalyst keeps working after the light is turned off, disinfecting water even in the dark.

It has long been known that irradiating water with high-intensity ultraviolet light kills bacteria. Some water filters made for campers and hikers, for example, use this technology. Researchers have been working to enhance the method's effectiveness by adding a photocatalyst that gets activated by UV light and generates reactive chemical compounds that break down microbes into carbon dioxide and water.

The new photocatalyst improves on that by using visible, rather than UV, light. Synthesized by Jian-Ku Shang, professor of materials science and engineering at the University of Illinois, Urbana-Champaign, and his colleagues, the photocatalyst works with light in the visible spectrum--wavelengths between 400 and 550 nanometers. It consists of fibers of titanium oxide--a common material used as a white pigment--doped with nitrogen to make it absorb visible light. Alone, the nitrogen-doped titanium oxide kills bacteria, though not efficiently. The researchers added nanoparticles of palladium to the surface of the fibers, greatly increasing the efficiency of the disinfection. He and his colleagues at the Shenyang National Laboratory for Materials Sciences in China published their work online in the Journal of Materials Chemistry.

"It would be very nice to shift activity of the traditional [photocatalyst] materials, which were only activated by ultraviolet radiation, to visible," says Alexander Orlov, assistant professor of materials science and engineering at Stony Brook University in New York. "If you look at the solar spectra, it contains only 5 percent ultraviolet and around 46 of visible." Such photocatalysts would allow solar energy to be used more efficiently as well as used indoors, since fluorescent lighting contains very little ultraviolet light.

Shang and his colleagues tested the photocatalyst by placing it in a solution containing a high concentration of E. coli bacteria and then shining a halogen desk lamp on the solution for varying lengths of time. After an hour, the concentration of bacteria dropped from 10 million cells per liter to just one cell per 10,000 liters.